The install is straight forward from the directions you get with the gauge. If you decide to put the sensor "inline" the rubber hoses like I did then you need a sensor manifold like the one I got from Dakota Digital because it has a screw where you can add a ground, without the ground the sensor will not work. I placed the sensor between the line that comes out of the transmission to the radiator (passenger side) because this is the hottest point of the trans fluid. Others like to place theirs on the return side. Its personal preference. I chose this side because the 99-02 4runners have a built in temp sensor and that is where they placed it. 96-98s did not come with that sensor.
Besides buying the manifold (block) from Dakota Digital, you have to buy (2) brass fittings. The specs are 1/4" Male NPT to 3/8" Barbs. The fittings the manifold comes with are compression fittings for metal tubes. Other then that the Autometer temp gauge comes with the 1/8" brass sensor that screws into the manifold. That's pretty much it besides running basic wires and mounting the gauge. A lot of guys make their own manifolds from brass fittings you get from hardware stores but you still need a ground. For 35 bucks I just went with the Dakota Digital.
Transmission Temperature Sender